Phase-shifting circuits are used in many electronic applications, such as in oscillator, phase-locked loops, and filters. RF and microwave frequency phase shift circuits are also useful as components in antenna array systems. The utility of phase-shifting circuits will be illustrated below, using the antenna array as an example.
As noted in U.S. Pat. No. 4,105,959, invented by V. Stachejko, a microwave phase shifter is a device that is capable of changing its electrical length, or phase, in a predictable manner in response to a proper command signal. In a phased array radar system, the electrical length of transmission lines interconnecting parts of the system is critical. For purposes of regulating the phase of transmitted or received signals, the electrical length of transmission lines between a transceiver and the several radiating elements in the antenna array must be made to vary. A typical system requires, for example, several antennae and phase shifters.
Microwave phase shifters were conventional fabricated using either diodes or ferrites as the switched material, using coaxial, stripline, microstrip, or waveguide construction. Several types of diode phase shifters have been devised such as switched line, hybrid coupled, loaded line, and three element “π.” or “T” circuits. In particular, the hybrid coupled circuit includes a 3-decibel (db)-quadrature hybrid with a pair of balanced diode switches connected to identical split arms of the hybrid. The hybrid coupler has been used extensively because it achieves larger phase shifts while using only two diodes.
One of the undesirable characteristics of the hybrid coupled diode phase shifter is the unbalance in the insertion loss, as the diodes are switched between the phase states. This unbalance results from the difference in loss produced by the diodes in the conducting state (“on”) and the non-conducting state (“off”). The insertion loss is a measure of the change in power (amplitude) between the RF input and output of the phase shifter. It is desirable that the amplitude of the outputs be the same as that of the input and, thus, the difference in insertion loss between the diode switching states be zero. A phase shifter having substantially equal insertion loss between the phase shifter states is desirable for wide frequency band operation. Accurate steering of an antenna beam in a radar system, for example, requires that the phase and amplitude errors be kept small.
Phase shifters made from inductor-capacitor (LC) components suffer from similar problems. As in an LC filter, minimum insertion loss is obtained at the resonant frequency. Changes in phase can be obtained by shifting the resonance away from signal frequency. However, a shift in the signal phase necessarily is accompanied with a change in the signal insertion loss. Thus, LC circuits are prone to the same problems as the above-mentioned diode phase shifters.
With the advent of digital technology, analog-to-digital (A/D) processing techniques have been applied to solve problems associated with the use of the above-mentioned passive and diode components. As noted in U.S. Pat. No. 6,784,831, Wang et al., the signals received from a number of antenna elements can be supplied to signal processing channels, which provide a variable gain and variable phase shift to such signals. An antenna pattern for the combined receive signal can be formed by a set of specific gain values and phase shift values over the signal processing channels, and a specific geometry and placement of the N antenna elements. The set of specific gain values and phase shift values is commonly referred to as “weights” (or “weight vector”) for the phased array antenna system. A unique advantage of the phased array antenna system is that the antenna pattern can be adjusted by changing the “weights” to perform one or both of the following operations:
a) beam steering: steering the beam by adjusting the phase shift values of the pattern for each processing channel; no adjustment to the gain values of the pattern is necessary; or
b) antenna null: the phase shift values and gain values of the pattern are adjusted to the suppress signal (i.e., interference) from a specific direction.
One form of architecture utilizes a large number of analog-to-digital converters (e.g., one for each antenna element), which substantially increases the cost of the system. Another drawback is that the input signal level of the analog-to-digital converter needs to be at a substantially higher level as compared to that of the received signal at the antenna element. Thus, the received signal needs to be amplified by one or more stages of amplifiers in order to bring the received signal to a level that the analog-to-digital converter can operate properly. Such multistage amplification increases the cost of the system. Yet another drawback of this architecture is that is difficult to maintain the signal delays precisely through the channels because the number of processing elements between the antenna element and the analog-to-digital converter is high. Such precise signal delays are required for accurate beam steering and nulling operations. Thus, calibration of these signal delays is required, which limits the suitability of this prior art phased array antenna system in many wireless communication applications. Another problem, at least with portable handsets, is that there may not necessarily be enough space to house, or power to operate all these components.
Many wireless communication receivers suffer from multipath errors, which are caused by the receiver receiving a composite of the direct signals, and reflected signals from nearby objects such as the ground or nearby buildings. The occurrence of such multipath errors is common within cities with high-rise buildings. A typical antenna can receive-both direct line of sight (LOS) signals and multipath signals. The direct line of sight (LOS) signal and the multipath signal are summed according to their relative phase and strength, resulting in a composite signal having a timing epoch that is different from that of the direct line of sight (LOS) signals. The receiver may be incapable of distinguishing and “rejecting” the reflected signal from the direct line of sight (LOS) signal unless the signal propagation delays of the two signals differ substantially. Beam steering operations can potentially provide antenna gain to reinforce direct signals and nulling operations are used to minimize the effect of received multipath signals. The drawbacks of the architecture of this phased array antenna system as described above also limit its suitability in addressing multipath rejection. More specifically, precise signal delays are required for accurate beam steering/nulling operations that are required for effective multipath rejection. Thus, calibration of these signal delays is required, which limits the suitability of this prior art phased array antenna system in many applications where the user is not realistically expected to maintain and calibrate the equipment.
It would be advantageous if a relatively simple phase-shifter circuit could be devised that could provide a uniform gain at every signal frequency.